As light sources for illumination apparatuses, incandescent bulbs and fluorescent lamps have been conventionally used. For illumination apparatuses that are required to output illumination light with high brightness, mercury lamps, halogen lamps, and xenon lamps, etc., are utilized as light sources. For example, Japanese Unexamined Patent Publication No. 2004-264512A discloses an illumination apparatus using an ultrahigh pressure mercury lamp and a projection type image display apparatus using the illumination apparatus. Further, recently, illumination apparatuses using LEDs as light sources have become popular. The service life of LEDs is generally ten thousand hours or more, and LEDs have gained attention as new general-purpose light sources that are replacing incandescent bulbs and fluorescent lamps.
On the other hand, laser light sources are widely utilized as light sources that generate coherent light in various industrial fields. In detail, laser light has been widely utilized in the optical communications field, the semiconductor device manufacturing field, and the optical field of holograms, etc., by taking advantage of its unique features such as monochromaticity, directionality, and coherency. In particular, semiconductor lasers have sufficiently long service lives as practical light sources, and also have extremely high energy use efficiency. The reason why illumination apparatuses using laser light sources have not generally become popular so far although they have the above-described advantages is that laser light sources have a unique problem of occurrence of speckle noise.
Speckles form a spot-like pattern that appears when coherent light such as laser light is irradiated onto a diffusing surface, and is observed as spot-like unevenness in brightness on an irradiated surface when laser light is irradiated onto an illuminating object. For example, when one point on a screen is indicated with a laser pointer, the spot of the laser light appears as a bright glare on the screen. This is caused by the occurrence of speckle noise on the screen, and the speckle noise becomes a factor that physiologically causes harmful effects to an observer. It is considered that the reason why speckles are caused when coherent light is used is that coherent lights reflected by portions of the diffusing and reflecting surface of a screen, etc., interfere with each other due to extremely high coherency. For example, detailed theoretical consideration on occurrence of speckles is described in “Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006.”
In use as a laser pointer or the like, a small spot is only seen by an observer, so that a severe problem does not occur. However, when a laser light source is used as a light source of an illumination apparatus and a comparatively wide region is illuminated by coherent light, speckle noise occurring in the illuminated region physiologically harmfully effects an observer and causes symptoms such as feeling sick. Because of this problem, illumination apparatuses using laser light sources are conventionally recognized as unsuitable for general use.
Of course, several detailed methods for reducing the above-described speckle noise have been proposed. For example, Japanese Unexamined Patent Publication No. H06-208089A discloses a technology by which laser light is irradiated onto a scatter plate, scattered light obtained from the scatter plate is utilized as illumination light, and the scatter plate is driven to rotate by a motor, whereby reducing speckles. Japanese Unexamined Patent Publication No. 2004-144936A discloses a technology for reducing speckles by oscillating an optical diffusing element disposed between a laser light source and an illuminating object. However, to rotate a scatter plate or oscillate an optical diffusing element, a large-scale mechanical drive mechanism is necessary, so that the apparatus is entirely increased in size, and power consumption is also increased. Speckles are not necessarily effectively removed by these methods.
Therefore, an object of the present invention is to provide a technology for efficiently and sufficiently reducing occurrence of speckles in an illumination apparatus using a coherent light source.